The present invention relates to a new electrode which can be used in electrolytic cells serving for the production of chlorine, of caustic soda or of chlorates, and a new method of preparing said electrode. The cells serving for the production of chlorine and caustic soda are either diaphragm cells or mercury cells. The chlorates are produced in a cell whose structure is similar to that of the diaphragm cells but which, however, has no diaphragm.
The electrodes generally used as anode in these cells are frequently made of graphite. Their use has always entailed certain drawbacks resulting from their wear, causing an increase in the voltage necessary for the satisfactory operation of the electrolysis cell as the result of the consequent increase in the distance between anodes and cathodes and the contamination of the electrolyte.
Work has been directed to the development of anodes from a metal having good resistance to corrosion of the electrolyte and covered with an electrochemically active precious metal, the resulting composite then being subjected to a treatment which favors activation. These anodes are dimensionally stable and do not have the drawbacks mentioned above. For example, there have been proposed anodes having a core of zirconium, zirconium-titanium alloy, tantalum or niobium, covered with platinum. There has also been proposed a titanium anode covered with platinum. Titanium, like the other core metals mentioned above, being a barrier or film-forming metal is capable of forming a barrier or film layer of oxide in the electrolysis solutions in order to protect its surface from corrosion at the places where the platinum is porous.
Also, electrodes have been made of one of these film-forming metals or alloys capable of forming a barrier or oxide film layer, and covered with a precious metal oxide or mixtures of oxides of precious and non-precious metals.
Another type of electrode has also been described in the literature, which electrode comprises a substrate of a barrier or film-forming metal, such as titanium, covered with a surface of a perovskite. A perovskite is an oxygen compound of two different metals which is well known in the literature and may be represented by the formula: EQU A.sup.a B.sup.b O.sub.3
in which A represents one metal ion and B another metal ion. A and B are related by the equation a + b = 6 in which a and b represent the conventional valences or ionic charges of the ions A and B, respectively. A discussion of the structure of perovskites is to be found, for instance, in "Crystal Structure" by Wyckoff, Vol. 2, 2nd edition (1964), Wiley and Sons, pages 390 to 402.
The drawback of these electroconductive layers of perovskites is that they are characterized by a high temperature of formation, greater than 800.degree. C., and more particularly between 1,000.degree. C. and 1,700.degree. C., which does not permit the depositing in situ of these products on a metal support without damage to the latter.
The processes for the deposition of these mixed oxide perovskites generally encountered up to the present time consist in depositing the perovskite via an organic or inorganic binder on the core metal substrate. However, it is known that in order to improve the conductive properties of the electrode, it is necessary to interpose between the substrate and the perovskite an intermediate layer which is more resistant to oxidation under the anodic conditions than the substrate is.
The best results up to the present have been obtained by depositing between the substrate and the layer of perovskite, an intermediate layer of a metal of the platinum group, such as, for instance, rhodium, osmium, iridium or platinum or their alloys or an oxide of the metals of the platinum group. The electrodes obtained in this manner, although having good electrocatalytic properties, are not entirely satisfactory, since even greater thicknesses of precious metal are necessary to obtain the desired result. This makes the process uneconomical.
It is, accordingly, an object of the present invention to provide an improved electrolysis electrode and process for producing it which do not have the disadvantages of the prior art.
It is another object of the present invention to provide an improved electrode for an electrolytic cell employing a perovskite compound.
It is also an object of the present invention to provide an improved method of producing an electrode for an electrolytic cell, which electrode is produced from a perovskite compound.
Other objects will be apparent to those skilled in the art from the present description.